Thursday, May 10, 2007

Aqueducts

I'm currently taking a class on Roman aqueducts. We discuss and research the aqueducts that supplied Rome (and other portions of the Roman empire) from a number of different perspectives, including historical, hydrological, cultural, economic, logistical, etc. Basically, it's a class covering a broad range of topics pertaining to Rome's water infrastructure, with the aqueducts at the core of discussion.

As part of the class, we have to do research on a topic of our choice. Originally, I wanted to do research on how the aqueducts reduced the instance of water-borne disease, but my preliminary research revealed a rudely obvious fact: getting data on that subject was going to be close to impossible. I'd also made some crude assumptions, such as assuming that water-borne diseases like Cholera that produced epidemics during the 19th century were present (they were not--Cholera was endemic to the Indian subcontinent until the 18th and 19th centuries). Basically, it was doomed from the get-go.

With that fresh defeat behind me, I got to thinking--how did the water supply effect the population of Rome? How did the population of Rome effect the water supply? The aqueducts were built over a span of 500 years, so one thing I plan on examining is how the water supply grew in relationship to the population growth of Rome during 312 B.C. and A.D. 200. I'd like to attempt to answer whether or not new capacity was added in response to demand, and if the added water in turn sparked increased population growth.

Admittedly, this task is also fraught with peril: gigantic discrepancies in the capacity of the aqueducts exist in current research. Archaeological evidence and historical accounts of their capacity are obviously suspect, because stuff generally isn't built to last 2000+ years (even the Romans, notorious for their over-engineering, aren't up to snuff), and historical records are sparse and sometimes cryptic.

Another huge stumbling block is the actual population of Rome--again, estimates vary greatly, from a peak population of 200,000 inhabitants (absurdly low) to 2,000,000+ (absurdly high). The general consensus among reading seems to be around a million, but it's an exceptionally wide range. Also unknown is the rate of growth, epidemics that may have caused lapses in growth, immigration, the effects of being sacked seven or eight times, famines, etc. The demographic information leaves much to be desired.

Still, the estimates are good enough to attempt to draw some conclusions. And the idea sounds entertaining to me, so...without further ado:

...the above is a chronological listing of Rome's aqueducts. I have the dates as negative numbers because I wanted to do some basic graphing in Excel. Anyways, this chart shows their rough completion date, overall length, altitude of water source, altitude of the water level in Rome, and the total portion of the aqueduct that was above ground. Much of this information is from the meticulous records of a model public servant by the name of Sextus Julius Frontinus, from which most of the "hard" data today is derived. Because Frontinus died before the completion of the Aqua Traiana and the Aqua Alexandrina, little is known about those aqueducts other than some scant archaeological evidence.

Here is another chart, showing the "output" (in quotes for a damn good reason--more on this in a bit) of each aqueduct in quinariae, according to several sources:

Overall B&H is "Blackman and Hodge," who had estimates based around modified figures derived from Frontinus' records. Hodge later tabulated greatly increased values in a subsequent publication, hence the separate column. The last column are figures from Gerda de Kleijn, who also made estimates based on modified figures from Frontinus.

The first thing to notice about this table is the vast discrepancy between the last four columns. All four columns represent estimates by published sources. I'm going to do more research into this discrepancy, but for now, I'll let it be. The second thing to notice is that these units are all in quinariae.

Much of the controversy over the total water "output" supplied by the aqueducts stems around the rather nebulous nature of a quinaria. A quinaria was a Roman unit of area, roughly equal to 4.2 square centimeters. Its primary use was to measure the cross-sectional area of pipes in Roman water distribution systems. Even in Roman times, there was controversy over the value of a quinaria--according to Frontinus:

Those who refer (the quinaria) to Vitruvius and the plumbers, declare that it was so named from the fact that a flat sheet of lead 5 digits wide, made up into a round pipe, forms this ajutage. But this is indefinite, because the plate, when made up into a round shape, will be extended on the exterior surface and contracted on the interior surface. The most probable explanation is that the quinaria received its name from having a diameter of 5/4 of a digit...

...in other words, Frontinus disagreed with Vitruvius about the actual value of a quinaria (primarily on the basis of Vitruvius' estimate being "imprecise" with respect to inner and outer circumference). Regardless of the Romans haggling over the value of their own units, one thing is certain: using quinariae to measure flow would be somewhat like using feet to measure acres, or kilograms to express newtons. The Romans did have units of volume, but the issue with the water flow is that it's a measurement of volume over time, which the Romans had no accurate or practical means of measuring. Culleus per elapsed sundial just doesn't roll of the tongue quite like cubic meters per second. Considering they had no accurate temporal measuring devices, it's debatable if the Romans even grasped the concept of units expressed over time.

To get a better sense of how the flow of water changed over time, I did some graphing in Excel:

The x-axis is time, and the y-axis is cumulative water flow in quinariae. The different colors correspond with a specific aqueduct. This is my first stab at graphing some of these data, so I plan on doing much more here; I want the temporal axis to be to scale, rather than the regular intervals in the above graph, but Excel seems suited only for the most basic charts. I suspect I'm going to have to draw much of these by hand.

Here's another interesting graph that shows how dependent Rome was on each aqueduct over time:

...considering the scale of aqueduct construction, new projects were not to be taken lightly. So there must have been a practical reason to add new capacity--could it be population? Who knows. :)